Low distortion signal band shifting with on-off switches

ABSTRACT

Simple switches are used for shifting a signal band, fo, to a new location, fo+f1, without distortion. The band is split into two opposed phases, chopped with square waves of frequencies f1 and odd harmonies 3f1, 5f1, etc., which are weighted in amplitude by the factors 1, one-third, one-fifth, etc., and are finally added. Unwanted components 3f1, 5f1, etc., are found to be easily canceled, obviating a difficult filtering problem.

llnile Mame 1en1 Inventor Lawrence 111. Weill 455 Poinl Lama Ave, San Diego, Cnllll. 921107 Appl. No. 733,463

Filed May 31, 11966 Patented Dec. 114, 19711 Wll'llllll ON-UIFIF Sill/11111311111516 7 11111111111115, 3 Drawing l igs.

111.5. 1131 325/435, 325/436, 325/437, 325/442, 328/16, 328/167 1m. 1C1 11-11041: 1/26, 1103b 1/04 Fiehl oil Seer-elm 325/430,

LOCAL OSCILLATOR 1 [56] lllellelrenees Clled UNITED STATES PATENTS 1,647,609 11/1927 Cotter 325/434 2,621,289 12/1952 Gray 325/430 3,399,278 8/1968 Dahlman 179/15 3,411,110 11/1968 Walker 332/17 Primary Examiner-Richard Murray Assistant Examiner-James A. Brodsky All0rneys- Louis A. Miller, Paul N. Critchlow, John W,

McLaren and Truman L. Styner AlBS'lllllflMC'll: Simple switches are used for shifting a signal band,f,,, to a new location,f,,+f,, without distortion The band is split into two opposed phases, chopped with square waves of frequencies f and odd harmonies 3}}, 5f,, etc., which are weighted in amplitude by the factors 1, onelhird, one-fifth, etc., and are finally addedv Unwanted components 3f,, 5],, etc., are found to be easily canceled, obviating a difficult fil' tering problem.

BAND PASS FlLTER OUT Patented Dec. 14, 19

2 Sheets-Sheet 1 1m ,8 22 m X A a c I I P F1 LOCAL I OSCILLATOR fr 3f, 1

F/ G I BAND PASS f FILTER I T I fl A M SWITCH q T I POSITION 3"? B 1 TIMING DIAGRAM F/ G 2 INVIiNTUR,

LAW/FENCE R WE/LL 11 0 NEYS 2 Sheets-Sheet 2 A/SFECTRUM OF SWITCH A SQUARE WAVE FREQUENCY 'f A x 0 w sr, f 3fff 5f f 5W 7f f 7f +f 9f -f I 1 I 1 I SWITCH a OLIJTPUT 5B SPECTRUM 0 I \F/ 4' \V 9M 9 em I O l 0 SWiTCH c OUTPUT 5G SPECTIRUM I 0 i w W 5f 5f -f 5f +f I OUTPUT SIGNAL FILTER CHARACTERISTIC FOR UPWARDS FREQ. SHIFT a m 7f f Tf +f SPECTRUM 0F FILTER OUTPUT FREQUENCY -9 INVIJNTOR. LA FENCE R. WE/LL BY 4. M

477' RNEYS LOW DISTORTIION SIGNAL BAND Sll-llllF'llllNG WllTllll N- UFIF SWITCHES The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND The process of heterodyning is commonly used in radio telemetry and other forms of data transmission to frequencyshift the frequency spectrum of time'varying signals from one spectral region to another. Preservation of the spectral characteristics requires the high-accuracy analog multiplier for combining the signal spectrum with a locally generated frequency. Even though analog multipliers may be quite expensive they do not always have the required accuracy. In sonar signals, for example, it may be necessary to analyze the signal spectrum for an obscure ships signature," and the spectrum might be shifted from some subaudible range to a higher frequency range without losing any of the information in the spectrum. This means that there can be no distortion in either phase or frequencywise. In another case, signals on tapes running at different speeds may be shifted to a common time base to permit processing in a correlator. Whether the frequency shift is large or small, there is the ever present problem of crossover of sidebands produced by different multiples of the local oscillator frequency.

The object of this invention is to provide an improved heterodyne that will shift a signal spectrum without phase or frequency distortion and which is inexpensive to make and simple to operate.

SUMMARY A band of signal frequencies having a center frequencyfl, is split in phase to produce two phase'opposed signal bands in separate circuits. A local wave of frequencyf which is equal to the desired shift, is generated as well as odd numbered multiples, 3f,, etc., and each frequency drives a switch that connects alternately the phase-opposed signals to the output circuit after they have been weighted in amplitude by factors, respectively, of l onethird, one-fifth, etc. Unwanted sideband components within the wanted spectrum as effectively canceled by adding to the output a phase-opposed equivalent of the unwanted component. Frequency filtering is thus obviated.

Other objects and features ofthis invention will become apparent to those skilled in the art by referring to the preferred embodiments described in the following specification and shown in the accompanying drawing in which:

FIG. 1 shows a schematic circuit diagram of one embodiment of this invention.

FIG. 2 is a switch position timing diagram of the switches of H0. 1, and

FIG. 3 shows spectrum diagrams on a common time base of principal signal voltages in the system of FIG. 1.

While it is contemplated that the switching circuits of FIG. ll should be solid state devices, the equivalent electromechanical switches are illustrated for ease of description. As will appear, the input signal spectrum at input terminal with a center frequencyfl, will be shifted in frequency by an amount f and applied to the output terminal 12. That is, the spectrum with center frequencyfl, shown on the top line of FIG. 3, is to be shifted to f l-j}. shown on the bottom line of FIG. 3.

According to this invention. the local frequency f is generated by the clock generator M, FIG. l, and is square or binary in nature. As shown in FIG. ll, the frequency is increased by integral multiples in multiplier 16. The multiples of a square wave of frequency I of interest here are odd num bered, 3f,, 5f etc. Although square wave switching of the signal preserve the spectral content of the signal to a high degree, because electronic switching can be made very nearly ideal, there are disadvantages to this technique of heterodyning. Spectral overlay occurs under certain conditions as is shown in FIG. 3 where sidebands created by at least one multiple of the local oscillator may overlap the desired band at the output. When the desired and undesired frequencies overlap, no amount of filtering will isolate the wanted signals. This overlay occurs whenever an integral multiple of the shifting frequency f, falls inside the signal spectrum.

According to this invention, the problem of overlap is eliminated while retaining the advantages of the low-cost simple switching type mixer.

The input signal is applied to a phase splitter such as the amplifiers l8 and 20 with positive and negative gain, respectively, to divide the signal and apply to bus bars 22 and 24 phase-opposed replicas of the input spectrum. Each bus bar is connected to like contracts of the single pole, double throw switches A, B, C, and D. The actuating electromagnets for the several armatures of the switches are driven, respectively, at frequencies f,, 31",, 5f,, and 7f,. Conveniently, these related frequencies are obtained from multiplier 16 connected to the local oscillator or clock generator M. If desired the frequency of the local oscillator may be well above the range of switching frequencies and a binary divider used at to.

The output of each vibrating switch is applied in appropriate proportion to the operational amplifier 26. That is, the output of each switch A, B, C, and D is appropriately attenuated and additively applied to the input of the amplifier. The attenuation is effected in resistances or linear impedances R, 3R, SR, and 7R. The amplifier 26 is of the type employing considerable negative feedback so as to simulate infinite or very high-input impedance and reliably perform analog summations of applied voltages from many inputs.

In considering the operation of the circuits of FIG. 1, reference should be made to the waveforms of FIG. 3. It will be assumed that the switch structures are ideal in producing the square-cornered wave of FIG. 2. It further will be assumed switch A is vibrated at frequencyf and thatf falls within the boundaries of the input signal spectrum to be shifted. The input I is centered at f,,. A square wave of frequencyf added as a modulation component to the signal is certain to produce modulation products containing odd-numbered multiples or harmonics off such as 3f,, 5f,, etc. It has been shown that the amplitudes of the harmonic components are, respectively, one-third, onefifth, etc. of the fundamentalf as shown on the top line of FIG. 3.

Unfortunately, each harmonic produces a pair of sidebands with the input signal. As shown on line 5A of FIG. 3, several harmonics are combined with the input signal to produce upper and lower sidebands. If the wanted band isf +f,,, it will be overlapped by the lower sideband, 3f,fi,, of the first harmonic.

Lines 5B and 5C, respectively, show the spectrum at the output of switches B and C. The spectra of switches B and C are inverted because the spectral components are I out of phase with the corresponding components of the output of switch A on line 5A. Note that at time zero, and each T, interval thereafter, in the square waves of FIG. 2, waves 18 and C are of one phase and opposite the phase of wave A. Reversing switches, not shown, may be provided in the leads to the electron inputs to facilitate selection of the: phase at each switch in accordance with the timing diagram, FIG. 2.

It is significant that the 3 f,-f,, component of the signal at the output of switch B can be made identical in amplitude with the corresponding opposed-phase component at the output of switch A, by the mere adjustment of the values of adding resistors R and 3R. Thus, the unwanted component 3f,-f., in the wanted output channel is perfectly canceled. The wanted channel f,+fl, in this case appears at the output terminal 12. All other undesired frequency components are well removed from the desired channel and are easily eliminated by simple filters. Although switch C is not required in the example shown, switch C as well as switch D were included to show that the elimination of troublesome higher order spectral components is merely a matter of adding switches driven at odd multiples of the rate of switch A. It is possible by adding enough switches to completely eliminate all higher order spectral components below any given frequency.

Mathematical analysis of the system of FIG. 1 becomes simple if it is assumed that the input signal, (1). is a cosinusoid given by S(!)=cos 21rfl,! l Such a simple expression can be extended to the case where the signal 3(1) is the sum of a large number of sinusoids of arbitrary frequency and phase.

Switch A of FIG. 1 effectively multiples S(l) by the function shown at A in the switch position timing diagram of FIG. 2. This switch function, f has the Fourier series representation where K is the number of the harmonic considered. Inspection of equation (2) of this series reveals that the spectrum of the square wave jl,(!) consists of frequencies that are odd multiples of the frequency f.. The amplitude of the nth harmonic of f is 4/n where n=l. 3. 5. 7. etc. When the input signal S(!) is multiplied by the square wavef.(1) there is obtained a scum-(c 2 ML; 2K-1 multiplying the terms yields For each harmonic number, K. in the summation, the two terms in the brackets represent upper and lower sidebands centered around the frequency odd-numbered multiple (2K 1 )f,. In a similar manner. there can be derived the output of switch B where, in the example of FIG. 1. K is 3 and the amplitude is one-third. As explained above it is preferred that the products of the square wave and the signal .i'(!) be attenuated in the adding resistors. R. 3R. 5R, etc.

What is claimed is:

l. The method of shifting a band of frequencies, having a center frequency f,,. from f,, to a new wanted spectrum centered atf,:tf,. said method comprising the steps of;

splitting the phase of said band to produce two phase-opposed replicas of said band in separate circuits.

locally simultaneously generating harmonically related phase coherent square waves of frequenciesf and nf.. where n is an odd integral number.

alternately feeding said phase-opposed replicas to an output circuit at said frequency f to produce modulation products];

alternately feeding said phase-opposed replicas to said output circuit at said frequency nf to produce modulation products "f 00s arm-um] weighting the amplitudes of the two modulation products.

respectively. by factors of l and l/n. and adding together the weighted signals so that unwanted higher-order spectral components of the mentioned '5 modulation products which overlap the wanted output spectrum are canceled. 2. A system for shifting a predetermined amount a band of signals having the center frequency, f,. to a new wanted band centered at f,;:f. said system comprising;

means for locally simultaneously generating a wave containing frequencyf and a predetermined integral multiple. "f thereof, where n is any odd whole number.

phase-splitting means for simultaneously producing in separate circuits opposed phases of said band of signal frequencies.

an output circuit.

means connected to the mentioned local generating means and responsive to said frequency f for alternately connecting said opposed phases from said separate circuits to said output circuit at frequencyf means connected to the mentioned local generating means and responsive to said frequency nf, for alternately connecting said opposed phases from said separate circuits to said output circuit at frequency "f and means for weighting the f -connected and the rif connected phases by factors. respectively. of I and Na. and for algebraically adding the weighted signals so that unwanted multiplication products appearing in the wanted output band are cancelled.

3. In the system defined in claim 2;

means for generating a plurality of odd-numbered integral multiples of frequency f., and

means responsive to each multiple for alternately switching said opposed phases to said output circuit.

4. In the system defined in claim 3;

means for attenuating each switched signal to an amplitude of l/n. where n is the number of said odd-numbered multiple.

5. In the system defined in claim 2, said local generating means comprising;

an oscillator for frequencyj], and

a multiplier for simultaneously generating a plurality ofoddnumbered multiples.

6. In the system defined in claim 2. said phase-splitting means comprising;

two amplifiers with inputs connected in parallel to said source of signals. and outputs connected. respectively. to said separate circuits. and

said amplifiers having gains. respectively. of +1 and -l to produce said opposed phase signals.

7. In the system defined in claim 2, said adding means comprising;

an operational amplifier having an apparent infinite input impedance. and

resistors connected between the amplifier input and.

respectively. fl-connecting and the nf,-connecting means, said resistors having. substantially, values of R and nR. 

1. The method of shifting a band of frequencies, having a center frequency fo, from fo to a new wanted spectrum centered at fo + OR - f1, said method comprising the steps of; splitting the phase of said band to produce two phase-opposed replicas of said band in separate circuits, locAlly simultaneously generating harmonically related phase coherent square waves of frequencies f1 and nf1, where n is an odd integral number, alternately feeding said phase-opposed replicas to an output circuit at said frequency f1 to produce modulation products f1 + OR - fo, alternately feeding said phase-opposed replicas to said output circuit at said frequency nf1, to produce modulation products nf1 + OR - fo, weighting the amplitudes of the two modulation products, respectively, by factors of 1 and 1/n, and adding together the weighted signals so that unwanted higherorder spectral components of the mentioned modulation products which overlap the wanted output spectrum are canceled.
 2. A system for shifting a predetermined amount a band of signals having the center frequency, fo, to a new wanted band centered at fo + or - f1 said system comprising; means for locally simultaneously generating a wave containing frequency f1 and a predetermined integral multiple, nf1, thereof, where n is any odd whole number, phase-splitting means for simultaneously producing in separate circuits opposed phases of said band of signal frequencies, an output circuit, means connected to the mentioned local generating means and responsive to said frequency f1 for alternately connecting said opposed phases from said separate circuits to said output circuit at frequency f1, means connected to the mentioned local generating means and responsive to said frequency nf1 for alternately connecting said opposed phases from said separate circuits to said output circuit at frequency nf1, and means for weighting the f1-connected and the nf1-connected phases by factors, respectively, of 1 and 1/n, and for algebraically adding the weighted signals so that unwanted multiplication products appearing in the wanted output band are cancelled.
 3. In the system defined in claim 2; means for generating a plurality of odd-numbered integral multiples of frequency f1, and means responsive to each multiple for alternately switching said opposed phases to said output circuit.
 4. In the system defined in claim 3; means for attenuating each switched signal to an amplitude of 1/n, where n is the number of said odd-numbered multiple.
 5. In the system defined in claim 2, said local generating means comprising; an oscillator for frequency f1, and a multiplier for simultaneously generating a plurality of odd-numbered multiples.
 6. In the system defined in claim 2, said phase-splitting means comprising; two amplifiers with inputs connected in parallel to said source of signals, and outputs connected, respectively, to said separate circuits, and said amplifiers having gains, respectively, of +1 and -1 to produce said opposed phase signals.
 7. In the system defined in claim 2, said adding means comprising; an operational amplifier having an apparent infinite input impedance, and resistors connected between the amplifier input and, respectively, f1-connecting and the nf1-connecting means, said resistors having, substantially, values of R and nR. 